RTR Shock Caps are really cool and help you build the shocks with no rebound.
Like Delves says, low friction O-rings are also cool.
don't forget Tungsten Carbide Diff and Thrust balls if you don't want to go down the gear diff route.
The Alloy rear wheel hexes can enable you to easily tune the forward drive
I'd stare with 0 offset but have a pair of 0.75mm offsets in my box for when I need a bit more.
Oh - and White Wheels. The practice ones in the box are at least a second a lap slower than white ones. Everyone except Griff knows that!!
